Flat-panel displays are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a display substrate to display images, graphics, or text. For example, liquid crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals and organic light-emitting diode (OLED) displays rely on passing current through a layer of organic material that glows in response to the electrical current. Inorganic light-emitting diodes (LEDs) are also used in displays.
Backlight systems can take a variety of forms. Direct-view backlights employ an array of light emitters located in layer behind a layer of light valves, such as liquid crystals. Edge-lit backlights employ an array of light emitters located around the periphery of a backlight. In either case, light diffusers are located between the light emitters and the light valves and other functional layers can provide functions such as light recycling, brightness enhancement, and polarization.
Originally, backlight systems employed small fluorescent light emitters that emit white light but more recently light-emitting diodes have provided an efficient alternative. Moreover, light-emitting diodes can produce relatively narrow-bandwidth colored light that is more efficiently transmitted through the color filters employed with light valve displays such as liquid crystal displays. In other embodiments, light-valve displays can be used with a color-sequential control scheme that renders color filters unnecessary. U.S. Patent Application Publication No. 20120320566, describes a liquid crystal display device and LED backlight system. EP 2078978 A3 discloses an LCD backlight containing an LED with adapted light emission and suitable color filters.
Backlit display systems typically suffer from reduced contrast ratio due to light leakage through the light valves and the limited on/off optical ratio imposed by light valves, especially the popular liquid crystal displays. To some extent, this problem can be mitigated with localized dimming. Localized dimming is accomplished by analyzing a display image, determining areas of light and dark in the image, and controlling light emitters in the corresponding area of the backlight to emit light in amounts corresponding to the luminance of the image areas. Localized dimming can be done separately for each color of light independently controlled in a backlight. Since light emitting diodes located in different areas of a backlight and that emit different colors of light can be separately controlled, backlights using light emitting-diode arrays can provide improved optical efficiency and contrast in a light-valve display. U.S. Pat. No. 8,581,827 entitled Backlight system and liquid crystal display having the same discloses a pulse width modulation control circuit to providing different brightness levels for adjacent rows of light emitters in a backlight.
However, light-emitting diodes are typically large, thereby increasing the thickness of a display and limiting the number of light emitters in a display area, and often are relatively less efficient at different brightness levels. For example, a direct-lit LED backlight unit for a high-definition display can have several hundred light-emitting elements and exhibit considerable blooming around bright spots in an image. Backlights using light-emitting diodes therefore limit display thinness and flexibility, are less efficient than is desired, and limit the extent to which local dimming can improve display contrast. Moreover, manufacturing processes for backlight systems using light-emitting diodes are relatively inefficient, requiring the placement of individual light emitters.
There remains a need, therefore, for a backlight having reduced thickness, improved electrical efficiency, improved display contrast, and improved manufacturing efficiency.